1. Field of the Invention
The present invention relates to color measurement instruments and, more specifically, to such instruments using light-emitting diodes (LEDs) as the illumination source.
2. Description of the Art
The ability to monitor and/or control color is beneficial in many areas of manufacturing. The performance requirements for measuring color from application to application differ greatly and the hardware solutions that result are perhaps as varied as the applications they serve. Nonetheless, a simple model can be posed where color measurement applications are to be thought of as falling somewhere along a simple linear Color Requirements Continuum (CRC) where the need for precision or accuracy is relatively high on one end and relatively low on the other end.
On the high end of the CRC are those processes where color is critical and must be scrutinized closely such as in printing, textiles, coatings, and film or digital image processing. In such applications, the colors to be measured or controlled are usually measured with precise instruments such as densitometers, spectrophotometers, or spectral radiometers. Such measurements may occur on-line or off-line, but in either case the instrumentation deployed tends to be precise, repeatable, and absolute rather than relative and specific to the application. Consequently such instrumentation is somewhat expensive.
On the low end of the CRC are those applications where some aspect of color is needed for a manufacturing process, but precision or absolute color is not a high requirement. These applications include such things as sorting items by color, or gauging some parameter of a process by color (e.g. watching for the crust to brown when baking bread). A variety of simple and robust RGB (Red, Green, Blue) color sensors generally serve this portion of the CRC. Such sensors usually differentiate color no finer than what could very easily be discerned by human vision in less than ideal lighting. The sensors are often used in situations where non-contact color measurement is required. These sensors tend to be more industrial in nature and often utilize an inexpensive PLC (Programmable Logic Controller) or RS-232 type of interface.
In the middle ground of the CRC, there is a demand for more precise color measurement, approaching the color discernment capabilities of instruments on the high end. Yet these applications cannot relax their requirement for environmental robustness, simplicity and reliability as demanded by industrial applications. Unfortunately, such applications often cannot fully justify the price of the industrially hardened high-end spectrophotometric solutions.
Existing approaches to low-cost industrial, sub-spectrophotometric color measurement fall into two general categories: (1) wide band illumination (i.e. white light) with the color differentiation occurring at the sensor, and (2) separate RGB illumination sources (to provide color differentiation) with detection performed by a simple wide band sensor. A common implementation of the first approach is exemplified by a pulsed xenon source and three filtered photodiodes. A common implementation of the second approach is exemplified by the Keyence technique of sequentially pulsing high brightness LEDs in red, green, and blue to illumine a sample that is then measured with a wide band sensor. Both of the above approaches seek to minimize the adverse affects of ambient lighting by using high levels of light at the instant of measurement or compensating for ambient light with differential measurements. The above techniques have served the low end of the CRC very well but continue to fall short of middle ground desires for precision, repeatability, or resolution in color discernment.